Page:Star Lore Of All Ages, 1911.pdf/246

174 mysterious depths of interstellar space. This wonderful region in the sky is known as "the Northern Coal Sack."

Cygnus is celebrated as containing the nearest lucid star in the northern hemisphere, a 5.6 magnitude star, barely visible to the naked eye, known as "61 Cygni." It is a double star, both being golden yellow in colour, situated six degrees north and east of the star ε Cygni. Bessel, in 1838, calculated its distance as approximately six light years,—a light year being the distance light travels in a year at the rate of 186,000 miles a second. 61 Cygni bears the distinction of being the first star whose distance was measured. If the distance from the earth to the sun equals one inch, the distance from the earth to 61 Cygni would equal seven and one half miles. This gives one a good idea of star distances, which to all intents and purposes are beyond our comprehension. To illustrate the amount of labour bestowed by astronomers on the problem of the determination of star distances, it may be mentioned that for the photographic measure of the star 61 Cygni, 330 separate plates were taken in the year 1886-7. On these, thirty thousand measurements were made. The result agreed closely with the best previous determination by Sir Robt. Ball, using the micrometer.

The lucida of the constellation is Alpha Cygni, called by the Arabs "Deneb," meaning the "Hen's Tail." It is a brilliant white 1.4 magnitude star, comparatively young, and in the same spectroscopic class as Spica and Vega. The spectroscope reveals that Deneb is approaching the earth at the rate of thirty miles a second. From the best determinations Deneb is at least ten times as far off as Vega. The distance of Vega is thus expressed: Supposing the sun's distance from the earth (93,000,000 miles) equals one foot, Vega is 158 miles distant. This puts Deneb at a reasonably safe distance from us.